Electrical gas discharge lamp

ABSTRACT

Electrical gas discharge lamps exhibiting an emission spectrum similar to the pure xenon molecular emission spectrum are characterized in that the gas in the lamps includes a mixture of xenon and argon the partial pressure of the xenon being less than 50 torr and constituting at least 50 parts per million of the gas mixture.

United States Patent 1 Gedanken et al.

ELECTRICAL GAS DISCHARGE LAMP Inventors: Aharon Gedanken, Nitzana 29,

Givatayim; Joshua Jortner, Beeri 50, Tel Aviv; Abraham Szoke, l-Iarakafot 22, Kfar Shmaryahu; Baruch Raz, Hofien 7, Ramat Aviv, all of Israel Filed: Jan. 24, 1972 Appl. N0.: 220,090

U.S. Cl 313/185, 313/209, 313/226 Int. Cl. H01j 61/16 Field of Search 313/226, 185, 209;

References Cited UNITED STATES PATENTS Faust et al. 331/945 G Dec. 11, 1973 OTHER PUBLICATIONS High-pressure Pulsed Xenon Laser By S. E. Schwarz et al. Applied Physics Letters, Volume l7, No. 7, October l970 pp. 305-306.

Primary ExaminerPalmer C. Demeo Attorney-Benjamin J. Barish [57] ABSTRACT Electrical gas discharge lamps exhibiting an emission spectrum similar to the pure xenon molecular emission spectrum are characterized in that the gas in the lamps includes a mixture of xenon and argon the partial pressure of the xenon being less than 50 torr and constituting at least 50 parts per million of the gas mixture.

5 Claims, 3 Drawing Figures I ELECTRICAL GAS DISCHARGE LAMP BACKGROUND OF THE INVENTION The present invention relates to electrical gas discharge lamps, and particularly to xenon lamps which have a strong molecular emission in the far ultraviolet range of about 1,700 A. The commercial xenon lamps ususally include pure xenon at a pressure of about 200 torr and since xenon is expensive, the cost of such lamps is relatively high.

SUMMARY OF THE INVENTION The present invention is based on the discovery that a continuous molecular emission can be produced with a very small fraction (as low as about one thousandth, and even less) of the amount of xenon used in commercial lamps, if the xenon is mixed with another rare gas, particularly argon. For example, very good results have been obtained when using only about 0.25 torr xenon mixed with about 300650 torr of argon.

This discovery was surprising and unexpected for several reasons:

First, it was surprising to find that a continuous molecular emission was produced with such small quantities of xenon, since pure xenon produces a continuous molecular emission only at pressure greater than about 50 torr, and even at this pressure, the continum is faint, the commercial lamps including about 200 torr as mentioned above. It was found that the xenon in the above described mixture of the present invention may constitut e as low as about 50 parts per million of the gas mixture.

In addition, it was found that the emission spectrum of this mixture has almost the same features as the pure xenon molecular emission spectrum, and not 'of that of the other rare gas used in the mixture, except that the peaks maximum is shifted A towards low energy.

Another surprising finding was that the emitted light was intensified when the xenon was diluted in the other rare gas; for example, a mixture including 0.25 torr xenon with 640 torr argon was found to have a substantially greater intensity than a mixture having the same amount of xenon and 320 torr argon.

The foregoing discoveries are believed to have substantial commerical importance because some of the rare gases are much less expensive than xenon; for example, argon costs about one-fiftieth the price of xenon. Thus, the invention enables the manufacture of gas discharge lamps of the xenon type (i.e. emitting the spectrum resembling xenon) at a substantially lower price than the presently available xenon lamps.

The invention thus provides a gas discharge lamp emitting light having an emission spectrum similar to the pure xenon molecular emission spectrum, the lamp including an envelope containing a gas and a pair of spaced discharge electrodes, characterized in that the gas is a mixture of xenon and argon, the xenon being at a partial pressure of less than 50 torr and constituting at least 50 parts per million of the xenon-argon mixture.

Preferably, the total pressure of the xenon-argon mixture in the lamp is about 200-800 torr, and the xenon constitutes about 0.1 percent thereof, that is, at a partial pressure of about 0.20.8 torr. The examples described below are based on an xenon-argon mixture having a total pressure of 640,430 and 320 torr, respectively, with the partial pressure of the xenon being 0.25 torr in all three mixtures.

Further features of the invention will be apparent from the description below.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a longitudinal sectional view of a lamp constructed in accordance with the invention and used for performing the experiments described below;

FIG. 2 illustrates the spectrum produced with a mixture of xenon (0.25 torr) and three different quantities of argon (640, 430, 320 torr); and

FIG. 3 illustrates two further spectra, one of a mixture of xenon and helium, and the other of a mixture of xenon in neon.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following preliminary discussion will be helpful in understanding the present invention.

It is known that all the rare gases emit a Hopfieldtype continuum peaked between 600 A and 1,700 A according to the gas considered. The continuum characteristic of the rare gas light sources originates from excited molecules. These molecules, having no bound states in the ground state, emit a broad Franck-condon determined continuum. These continua are shifted to lower energies with respect to the atomic resonance lines by the amount of the bindng energy of the molecule combined with the translational energy of the components in the ground state. In the case of Xenon this shift is about 1 eV. The light emitted is an afterglow with a duration of 10-60 p. sec determined by the emitting molecules.

Table 1 summarizes the relevant data available concerning the conditions of peration as well as useful emission ranges for the rare gases.

TABLE I Rare gas continua Useful range Pressure Gas range work(torr) He 580-1100 40-55 Ne 740- 1000 60 Ar 1050-1550 200 Kr 1275-1800 500 Xe 1 4802000 200 The present invntion' is based on the results of a series of experiments involving the excitation of low concentrations of xenon in other rare gases, particularly argon. It was shown that these mixtures emit a continum in the spectral region of 1,500 A 1,900 A. The peak of this continuum is at about 1,700 A. This emission is very similar to the pure xenon emission except the xenon amounts required are about one-thousandth that of pure xenon emission, or even less, the experiments indicating that the xenon could be as low as about 50 parts per million of the gas mixture.

The emission experiments were conducted in the modified Tanaka-type lamp illustrated in FIG. I. This lamp includes an outer envelope 2 of Pyrex formed with two electrodes 8, 10, each connected to a tungsten lead-in wire l2, 14. One end of the tube 16 was connected to a gas-supply system for supplying the gas mixture at the appropriate pressures, and the opposite end of the tube was closed by a lithium flouride (or sapphire) window 18. A pyrex tube 22 was fixed within the tube envelope 2 between the two electrodes 8, 10, which tube provided a path 22 along the longitudinal axis of the tube between the two electrodes for the excited gas mixture. Tube 20 also defines a compartment 24 separated from the gas mixture, which compartment was used for conducting a cooling fluid, namely water, the latter being introduced through outlet 26 and being removed from outlet 28.

In conducting the experiments describd below, special care was taken to ensure clean operating conditions which were secured especially by prolonged baking (for at least 24 hrs.) at 450C and 5X10 torr. As a purity check, the spectrum of a lamp containing the pure gases was taken. Only the a-Lyman (1,215.7 A) of hydrogen and the Cl (at 1,656 A) persisted in spite of the applied treatment. The high residual purity of the lamp was also due to the quality of the aluminum foil of which the electrodes 8,10 were produced. Its guaranteed purity was better than 99.999 percent.

The gases used were Matheson research grade. All the concentration adjustments were made on the lamp system in order to avoid contamination on transfer. The primary pressure was always lower than 10' torr.

All the measurements were conducted on a McPherson 225 monochromator. This is a 1 meter normal incidence apparatus into which a 1,200 lines/mm grating was installed. The resolving power was measured to be 0.13 A/100u slit width.

The spectrum of xenon in argon is shown in FIG. 2. This spectrum was taken while keeping the partial pressure of xenon constant (0.25 torr) and varying the partial pressure of argon. The argon pressures were: 320 torr, 430 torr and 640 torr, as shown by curves a, b and c, respectively. Slit width of 2 mm was used and the corresponding resolution was 25 A.

The two peaks observed at 1,296 A and 1,470 A are ascribed to emission of the xenon atom excited to the P, and P respectively. The broad emission peaked at 1,700 A cannot be attributed to any atomic emission. A very remarkable feature of this spectrum is he total absence of emission due to excited argon molecules.

Another remarkable behavior of the system is that an increase in argon partial pressure resulted in an increase of intensity of the peak at 1,700 A and a decrease in the atomic peak intensities.

FIG. 3 shows the results of essentially the same type of experiments in which helium (curve 11) and neon (curve 2) were used instead of argon. The spectra are very much of the same character as those in the case of argon.

Experiments with xenon-krypton mixtures did not produce evidence that a peak would be formed at about 1,700 A or under these partial pressures, but indicated that a much higher pressure would be required, in the order of five atmospheres, i.e. about 4,000 torr.

The most prominent feature of xenon in helium, neon and argon (and possible also in krypton but at substantially higher pressures) is the appearance of a broad emission peak extending from 1,550 A 1,850 A and having a maximum at 1,700 A. This emission peak has a striking resemblance to that of pure high pressure 200 torr xenon. On the other hand, it should be mentioned that pure xenon at a pressure of 0.25 torr, which was its partial pressure in the mixtures analyzed, does not give a continuum at all. At this pressure the emission in this region consisted rather of atomic resonance lines only. It should be concluded therefore that the pressure of perturbing gases such as argon, neon and helium enhances the production of excited xenon molecules.

This phenomenon can be explained in view of the recently published works on the formation of metastable excited xenon atoms. It has been shown that collisions can induce transitions from the P state of xenon (the transition of which to the ground state is allowed) into the not allowed 1, state. The lifetime of this level is of the order of magnitude of seconds, and this is ample time for the excited xenon to collide with another xenon atom in the ground state to form an excited molecule. The transition from this molecule to the ground state is allowed.

There is an experimental difference of 20 A between the maxima observed for the pure xenon continuum and the xenon mixtures continuum. The latter appearing at a lower energy. This energy different of about 700 cm corresponds quite closely to the 978 cm difference in the energies of the P and P generating states.

An additional observation in favor of the above outlined interpretation is the decrease of the intensity of the atomic resonance lines with increase of perturbing gas pressure (see FIG. 2). This increase in pressure also increases the rate of production of metastable states and thus favors the channel via molecular emission as compared to atomic emission.

Because of this similarity of the vacuum ultraviolet emission between the low concentration xenon rare gas mixtures and the pure high pressure xenon, electrical discharge lamps may be constructed including such xenon rare gas mixtures at considerably less cost than the present commerical lamps containing pure xenon.

Many variations, modifications and other applications of the illustrated embodiments will be apparent.

We claim:

1. A gas discharge lamp emitting light having an emission spectrum similar to the pure xenon molecular emission spectrum, said lamp including an envelope containing a gas and a pair of spaced discharge electrodes characterized in that said gas is a mixture of xenon and argon, the xenon being at a partial pressure of less than 50 torr and constituting at least 50 parts per million of the xenon-argon mixture.

2. A lamp as defined in claim 1, wherein the partial pressure of the xenon is from about 0.2-0.8 torr.

3. A lamp as defined in claim 2, wherein the partial pressure of the xenon is about 0.25 torr.

4. A lamp as defined in claim 1, wherein the total pressure of the xenon-argon mixture is about 200-800 torr.

5. A lamp as defined in claim 4, wherein the xenon constitutes about 0.1 percent of the xenon-argon gas mixture. 

2. A lamp as defined in claim 1, wherein the partial pressure of the xenon is from about 0.2-0.8 torr.
 3. A lamp as defined in claim 2, wherein the partial pressure of the xenon is about 0.25 torr.
 4. A lamp as defined in claim 1, wherein the total pressure of the xenon-argon mixture is about 200-800 torr.
 5. A lamp as defined in claim 4, wherein the xenon constitutes about 0.1 percent of the xenon-argon gas mixture. 